Aurora Australis

byJean TateonOctober 14, 2009

Aurora Australis over the elevated station at Amundsen-Scott South Pole Station, Antarctica. Credit: Calee Allen, National Science Foundation

Aurora australis (also known as the southern lights, and southern polar lights) is the southern hemisphere counterpart to the aurora borealis. In the sky, an aurora australis takes the shape of a curtain of light, or a sheet, or a diffuse glow; it most often is green, sometimes red, and occasionally other colors too.

Like its northern sibling, the aurora australis is strongest in an oval centered on the south magnetic pole. This is because they are the result of collisions between energetic electrons (sometimes also protons) and atoms and molecules in the upper atmosphere … and the electrons get their high energies by being accelerated by solar wind magnetic fields and the Earth’s magnetic field (the motions are complicated, but essentially the electrons spiral around the Earth’s magnetic field lines and ‘touch down’ near to where those lines become vertical).

So by far the best place to see aurorae in the southern hemisphere is Antarctica! Oh, and at night too. When the solar cycle is near its maximum, aurora australis are sometimes visible in New Zealand (especially the South Island), southern Australia (especially Tasmania), and southern Chile and Argentina (sometimes in South Africa too).

About the colors: the physics is similar to what make a flame orange-yellow when salt is added to it (i.e. specific atomic transitions in sodium atoms); green and red come from atomic oxygen; nitrogen ions and molecules make some pinkish-reds and blue-violet; and so on.

How high are aurorae? Typically 100 to 300 km (this is where green is usually seen, with red at the top), but sometimes as high as 500 km, and as low as 80 km (this requires particularly energetic particles, to penetrate so deep; if you see purple, the aurora is likely to be this low).

Aurorae on other planets? Well, as there are strong magnetic fields plus (not so strong) solar wind plus (really deep) atmosphere on Jupiter and Saturn, they have spectacular aurorae, in rings around their magnetic poles (which are closer to their rotation poles than Earth’s are). Aurorae have also been imaged on Venus, Mars, Uranus, Neptune, and even Io (atmosphere? solar wind? magnetic fields? sure, but very different than on planets).